Add a visual cue to indicate that stashing is in progress

**Is your feature request related to a problem? Please describe.**
When stashing it can take several seconds before the stash completes and the popup closes, but there is no visual cue that it is happening. I have on occasion wondered whether I actually pressed Enter and have pressed it again; fortunately, this seems to cause no harm.

**Describe the solution you'd like**
In the Stash popup, overwrite the "type name (optional)" message or the name (if typed) with a message like "Stashing..." as soon as Enter is pressed. This will inform the user that something is actually happening.